So, I used to do this sort of thing for a living - but not fraud or for the DWP
The Courts are not the place to sort out Social Security Issues, as is often pointed out by the Social Security and Child Support Tribunal.
So, what SHOULD happen is that the DWP needs to make a decision that you have been overpaid and the amount.
This decision is appealable.
Details of how to do it are at
https://www.gov.uk/appeal-benefit-decision
What the Tribunal will do is decide IF you have been overpaid, if it is repayable by you AND the amount of the overpayment which for various reasons may NOT be what the DWP says it is.
The Criminal Trial MUST NOT take place until the appeal has been heard, the reason being that until you have exhausted the Appeal process there is no confirmation that there is an overpayment, its amount and if it is recoverable.
The Upper Tribunal have made some pointed comments about the inability of the Criminal Courts to deal with benefits legislation
When the existence of the overpayment and its amount have been confirmed AND ONLY THEN can the criminal trial take place. Bear in mind that in the sentencing process the amount of the overpayment will be relevant.
You will need specialist benefit advice, I suggest your local Citizens Advice is a place to start or a Law Centre.
I am not qualified to comment on the behaviour of the DWP Investigators but those with a knowledge of Criminal Law may be able to comment
Failing that contact me